Recent advances in manufacturing techniques for producing solid-state lighting have led to an increase in the selection of wavelength and radiation intensity of LEDs. LEDs have several advantages that distinguish them from over conventional light sources. LEDs have stable lifetimes in the thousands of hours. In addition, LEDs have narrow-band spectral distributions which eliminate the need of a filter which is required when other conventional light sources are used in certain applications. LEDs are mechanically robust and less sensitive to physical and mechanical stresses which might occur during handling and transport. Such advantages of LED sources have been recognized with the development of integrating sphere sources which use LEDs that have many different center wavelengths for illumination as disclosed, for example, by S. W. Brown et al. (Development of a Tunable LED-Based Colorimetric Source, Journal of Research of National Institute of Standards and Technology, 107, pp. 363-371 (2002)). The ability to control the optical power of individual LEDs has led to a spectrally tunable source with a high degree of temporal stability.
Radiation thermometers are typically constructed with narrow-band filters. Since LED-based sources can be constructed to have common center wavelengths, such LED-based sources can be used as monochromatic light sources. These monochromatic sources that are based upon LEDs can be used to determine the size-of-source effect (SSE) which are typically only occasionally performed to characterize radiation thermometers as discussed by G. Machin et al. (A Comparative Study of Size-of-Source Effect Determination Techniques, Proceedings of Tempmeko 2001, Berlin, 2002, pp. 155-160) and H. W. Yoon et al. (Methods to Reduce the Size-of-Source Effect in Radiation Thermometers, submitted to the Proceedings of Tempmeko 2004, Croatia).
The present invention provides compact radiance sources based upon the use of LEDs that are much easier to use than large integrating sphere sources for determining the SSE. Since SSE has been found to be dependent on the scattering properties of the objectives of radiation thermometers, the SSE should be measured prior to any critical temperature determination. The radiance sources of the present invention allow for easy measurement of SSE and thus improvements in critical temperature determinations.
In addition, the LED-based radiance sources of the present invention provide stable and uniform radiance that can be used for the calibration and characterization of radiation thermometers, night vision devices, environmental remote sensing systems, biological agent detection, and any radiometer which needs a stable radiance source. In general, the LED-based radiance source can be uses as a replacement for lamp-based integrating sphere sources.